Process for recovering acid values from mixed acid waste



July 25A 19m J. o. DAsHER ETAL 2,993,757

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PROCESS FOR RECOVERING ACID VALUES FROM MIXED ACID WASTE Filed July '7, 1958 2 Sheets-Sheet 2 3 :ozon

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United States Patent Oce 2,993,757 Patented July 25, 1961 2,993,757 PROCESS FOR RECOVERING ACID VALUES FROM MIXED ACID WASTE John O. Dasher and David Goldstein, Pittsburgh, Pa., as-

signors to Crucible Steel Company of America, Pitts burgh, Pa., a corporation 'of New Jersey Filed July 7, 1955, Ser. No. 746,686 4 Claims. (Cl. 23-152) This invention relates to a process for recovering nitric and hydrouoric acid values from a waste pickle liquor solution and, more particularly, to a process in which distillation techniques are employed to recover the nitric and hydrofluoric acid values.

In the manufacture of metallic sheets of stainless steel and titanium, for example, it is necessary to pickle the product by immersion in a dilute acid bath containing nitric acid, a mixture of hydrouoric and nitric acids or any other mixture of acids suitable for pickling. In this process, the stainless steel is brightened and any defects on the surface of the metal sheets are removed by chemical action. After a predetermined amount of time, the build up of metal salts in the pickle liquor used in this process decreases the elfectiveness of the pickling action. Furthermore, the bath becomes diluted with water from condensation of steam which is normally used to heat the pickle bath. Consequently, it becomes necessary from time to time to withdraw the solution from the pickling tank and to replace it with a fresh solution.

Previous to this invention, it has been common practice to drain the waste acid solution into a lagoon located outside the plant site. This arrangement, however, is unsatisfactory since only limited amounts of acid can be handled in this manner and since the danger always exists that some of the acid will find its way into public waterways. .In addition, large amounts of the costly acids are lost and cannot be used again when this method of disposal is employed.

It is a primary object of this invention to provide a means for recovering acid values in a waste pickle liquor solution so that the only waste products produced are water containing small traces of acid and a sludge of precipitates. These Waste products may be easily disposed of while the recovered acids may be recycled to the pickling process.

More specifically, an object of this invention is to provide an acid recovery process of the type described above in which a dilute acid solution is distilled under vacuum conditions whereby lower temperatures may be used in the distillation procedure to prevent corrosion of the distillation equipment.

Still another object of the invention is to provide an Y acid recovery system in which sulfuric acid, is added to a pickle liquor solution containing a mixture of hydrouoric and nitric acids whereby the metal uorides and nitrates in solution are converted into sulfates which are insoluble in the concentrated sulfuric acid, while the fluoride and nitrate ions combine with the hydrogen ions from the sulfuric acid to form hydrouoric and nitric acids which are distilled olf.

In accordance with the invention hereinafter described, a solution resulting from pickling stainless steel or other alloys in a bath containing nitric and hydrouoric acids is irst distilled under vacuum conditions to drive off as much of the total liquid volume as possible without the loss of acid in the distillate becoming appreciable. From the rst distillation step, the distillate is passed to a second stage evaporator Where sulfuric acid is added to the liquor. In this step, the sulfate ion replaces the nitrate and iluoride ions to form metal sulfates Whichvprecipitate since they are substantially insoluble in concentratedsulfuric acid. The hydrouoric and nitric acids, havingahigher vapor pressure than sulfuric acid, boil oif along with the remaining water, are condensed, and recycled to the pickling tank. The sulfuric acid slurry containing a large portion of insoluble sulfates is then fed through suitable solid-liquid separation apparatus; and, the liquid portion consisting of at least 50% sulfuric acid is recycled to the second stage evaporator where it again forms insoluble sulfates while freeing all of the nitrates and iluorides as hydrofluoric and nitric acids.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification and in which:

FIGURE 1 is a ow sheet graphically illustrating the process of the invention; and

FIG. 2 is a graph illustrating one example of the distillation characteristics of a mixed acid pickle waste solution containing nitric and hydrofluoric acids.

Referring to FIG. l, a mixed acid waste at 1, consisting of hydrofluoric and nitric acids containing metal uorides and nitrates in solution, is fed through line A to a first stage evaporator B. As shown, the evaporator is heated by means of a coil C supplied with steam at 50 p.s.i.g. from line D. The evaporator B is operated under vacuum conditions (i.e., three inches of mercury) at F. Ati this temperature and pressure, the mixed acid product. will boil; whereas, if distillation took place under atmos pheric pressure, the temperature required would be approximately 22.5 F. One advantage of using the reduced pressure lies in the fact that as the mixed acid product is extremely corrosive, it is highly desirable to operate the distillation process at the lowest possible temperature. This is accomplished in the present invention by reducing the pressure as explained above. Another advantage of reduced pressure in the evaporator lies in the fact that as the pressure decreases, the ratio of the vapor pressure of water to that of the acids increases. Consequently, reducing the pressure facilitates the distillation of the water to a greater degree than that of the acids which remain in the residual liquid after this rst distillation step.

In FIG. 2, it can be seen that when the volume of mixed acid waste in evaporator B is distilled below 50% of its original volume for the example shown, the acid is desired to drive olf water primarily and retain the acid in the residual liquid, the distillation is controlled so that a maximurn of water is removed without substantial loss of acid. Although the 50% Ifigure is given for a particu-` lar sample of mixed acid waste, it is to be understood that the percentage may change upward `or downward, the controlling factor always being the point at which the additional acid loss is more important economically than obtaining a more concentrated acid product for reuse.

From the evaporator B, overhead vapor at 2 is fed toa barometric condenser E which is cooled by a spray from nozzle F supplied with cooling water from line G. The condensate produced from condenser E is then fed at 3 to a sewer. It will be noted that this condensate contains only 0.050% hydrofluoric acid and 0.1% nitric acid so that the product is not dangerous and may be easily handled. The non-condensables from condenser E' at'10 are fed to a two stage steam ejector system H containing a barometric intercondenser. This system is supplied, as shown, with steam and cooling water from lines D and G, respectively. The condensate from the ejector at- 11 is then combined with the condensate at 3 and passed on to a sewer. e

The concentrated liquor or residue from evaporator B at 4 consists of a mixture of hydrofluoric and nitric acids containing metal lluorides and nitrates in solution. This residue is fed to a'second stage evaporator I which, as shown, is operated under vacuum conditions (i.e., one inch .of mercury) and 150 F. As with evaporator B, evaporator J is operated at lthis reduced temperature to minimize corrosion problems. Sulfurie acid at 6 is fed through line L and filtration equipment M to recycle line N which leads to the evaporator I. The sulfuric acid fed into the evaporator J has a much lower vapor pressure than nitric or hydrouoric acid. Consequently, these latter two acids boil off while the sulfate ion in the sulfuric acid forms metal sulfates and releases the fluoride and nitrate ions which combine with the hydrogen from the sulfuric acid to forni more nitric and hydrofluoric acid.

The overhead vapor from evaporator I at 9, consisting of a mixture of hydrofluoric and nitric acids, is fed to a surface condenser P containing a cooling coil Q supplied with cooling water from line G. The condensate, consisting of a mixed acid product of 8.1% hydrofluoric acid and 22.9% nitric acid, then appears at 12. The noncondensable gases from the condenser at 13 are fed to a two stage steam ejector system S containing a barometric condenser. The condensate from the ejector S containing 0.14% H2804, 0.03% HF, and 0.11% HNO3 is nally passed to a sewer.

The residue from the second stage evaporator I at comprises a 25% solids slurry of sulfuric acid. This slurry is then passed to filtration equipment M where the liltrate, consisting of sulfuric acid, is recycled to the evaporator I. The iltration equipment M may, yfor ex ample, consist of a standard drum lter or any other apparatus which Will satisfactorily separate the solid and liquid portions of the slurry. Fresh sulfuric acid may be added to the system through line L and filtration equipment M as required. The fresh sulfuric acid serves as make-up acid and also serves to wash nitrates and iluorides from the iilter cake. In the iltration step, the concentration of the sulfuric acid must be at least 50% to prevent the sulfates from redissolving. If the make-up sulfuric acid required is greater than that which can be conveniently obtained in washing the cake, additional acid shouldbe added to the second stage evaporator I through a conduit, not shown, leading from supply line L. The filter cake at 7 is then disposed of in some suitable manner. One example of the composition of the lter cake is shown in FIG. 1.

Table I illustrates one example of continuous operating conditions of a system of the type shown in FIG. 1.

TABLE I Stream Stream Stream Stream Item Lb./day solid. Lb.,./day liqu` 84, 000 37, 800 S85, 702 46, 200 Denslty 1.1 vapoi 1. 1. 31 GaL/day.. 9, 000 100,300 4, 230 G.p.m 6. 3 60. 2. 9 LbL/day 100% HQSOi LbJday 100% IlF 4,040 (eq.) 505 407 4,135 Lb./day100% HNO3 12, 300 (eq.) 824 824 11,476

It Stream Stream Stream Stream Stream Lit/day 50nd 16,300 16,300 Lb. [day liquid 48, 900 36, 200 16, 300 68, 800 49, 800 (vapor) Density V1. 91 1.5 2. 2 1.6 Gah/day 4, 100 2, 900 5, 080 G.p.ni 2.0 2.0v 3.5 Lb./day 100% H2SO4 27, 000 21,700 9. 780 38,920 270 Lil/day 100% HF 475 93 (eq.) 382(eq..) 4, 042 lio/day 100%,HNO3. 62 62 (eq.) nil 11,489

N o'rE. (Eq.) signifies total tluorides and nitrates present as equivalent HF and HNOa.

In this table, the pounds of liquid or solid per day, the density, the gallons per day, and the gallons per minute 'are indicated at the various points in the system which are numbered 1 through 14 in FIG. l. It will be noted that about 3,986 pounds of hydrofluoric acid per day at point 12 may be produced from a mixed acid waste at point 1 containing 4,640 pounds per day of the same product. Similarly, over 11,000 pounds per day of 100% nitric acid are produced from l'12,300 pounds per day fed into the lirst Vstage evaporator. These figures Will, ,of course, vary as the pressure at which distillation takes place is changed. The invention thus provides an efficient and 10W cost means for recovering the nitric and hydrofluoric acid values.

In Table II, the effect of pressure on distillate losses in evaporator B is shown.

TABLE II Two-stage distillation tests (first st age) Original Distillate Losses l v Final Pressure Volume (Percent) Run N o. Temp. (p.s.i.a.) Distilled F.) (Percent) When atmospheric pressure is used, the distillate losses of the nitricand uoride ions accounts for about 3 to 4% and 4 to 6%, respectively. When, however, the pressure is reduced to l.8`p.s.i.a., the` distillate losses are reduced to 1.8% of the nitrate ion and 1.3% of ther fluoride ion respectively.

It can thus be seen that by conducting a two-stage vacuum distillation process in which a substantial portion of the original Volume of a mixed acid Waste, is distilled in the first stage, substantially all of the hydrofluoric and nitric acid values may be recovered.

Although the invention has been `described with particular reference to a mixture of hydrofluoric and nitric acids, it is to be understood that it is equally applicable to any acid rmixtu-re of acids in the group consisting of hydrotluoric (HF), nitric k(HNO3), hydrochloric (HC1), hydrobromic (HBr), hydriodic (HI), nitrous (HNOZ'),

or any other -relatively volatile acid which has a higher Y inated entirely since such a process would merely boil o a portion of the acid` values. In the second stage of distillation, salts precipitate whenV acid and water are boiled off. This precipitation is more complete as Ythe percentage of H2504 in solutionis increased until, at 70% H2504, it practically complete. 'Iflie4 precipitate islargelypnietal sulr'ates whichrhave little if any, water of hydration. The precipitate, however, is soluble in water except for small traces of calcium, lead, barium or' other rare elements which it may contain.

The process of the invention can be operated on a batch or continuous basis. If a batch process is employed, it is controlled by the distillation percentage in the first stage and by the sulfuric acid concentration in the second stage. If, however, the process is operated on a continuous basis, control is achieved by adjusting the flow rates of the slurry stream and acid.

The invention has been shown in connection with a certain specific embodiment. It Will be readily apparent to those skilled in the art, however, that various changes in form and arrangement of steps may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In the method of recovering nitric and hydrofluoric acid values from a waste pickle liquor solution containing nitrates and uorides in solution, the steps of: distilling the waste pickle liquor solution at about 150 F. and three inches of mercury to drive off a maximum of water without substantial `loss of acid radical, adding sulfuric acid to the concentrated liquor from said distillation step, further distilling the mixed product at 150 F. and one inch of mercury to drive olf nitric and hydrofluoric acid while forming a 25% solids slurry of sulfate salts in sulfuric acid, separating the sulfate salts from the sulfurie acid slurry, recycling the sulfuric acid recovered in the separation step through said last type mentioned distillation process, and condensing the vapors derived from the first and second distillation steps.

2. In the method of recovering at least one acid selected from the group consisting of hydrouoric, nitric, hydrochloric, hydrobrornic, hydriodic and nitrous acids from a dilute, aqueous solution containing a salt of the acid, the steps of: rst evaporating a maximum of water without substantial loss of acid radical, adding sulfuric acid to the concentrated liquor from the first distillation step, distilling the combined product to drive off at least the said one acid selected from the group consisting of hydrofluoric, nitric, hydrochloric, hydrobromic, hydriodic and nitrous acids While forming a slurry of sulfate salts in 6 x sulfuric acid, condensing the acids thus driven off in the distillation step, separating the sulfate salts from the suluric acid slurry, and recycling the sulfuric acid recovered in the separation step to the last-mentioned distillation step.

3. In the method of recovering at least one acid selected from the group consisting of hydrofluoric, nitric, hydrochloric, hydrobrornic, hydriodic and nitrous acids from a dilute, aqueous solution containing a salt of said acid, the steps of: distilling a maximum of water Without substantial loss of acid radical, adding sulfuric acid to the concentrated liquor from said distillation step, d-istilling the combined product under vacuum conditions to drive olf at least the said one acid selected from the group consisting of hydroiuoric,` nitric, hydrochloric, hydrobrorn-ic, hydriodic and nitrous acids while forming a slurry of sulfate salts in sulfuric acid, filtering the sulfate salts from the sulfuric acid slurry, and recycling the sulfuric acid recovered in the filtration step to the lastmentioned distillation step.

4. In the method of recovering at least one acid selected from the group consiting` of hydroiiuoric, nitric, hydrochloric, hydrobromic, hydriodic and nitrous acids from a waste pickle liquor solution containing a salt of said acid, the steps of: distilling the Waste pickle liquor solution under vacuum conditions to drive off a maximum of water without substantial loss of acid radical, adding sulfuric acid to the distilled solution, further distilling the combined product under vacuum conditions to drive off at least the said one acid selected from the group consisting of hydrofluoric, nitric, hydrochloric, hydrobrornic, hydriodic, and nitrous acids while forming a slurry of sulfate salts in sulfuric acid, and condensing the vapors formed in the last-mentioned distillation step to obtain a solution wherein the acid concentration is greater than in said pickle liquor solution.

References Cited in the le of this patent UNITED STATES PATENTS 336,822 Pool Feb. 23; 1886 1,130,104 Raschig Mar. 2, 19x15 1,857,346 Beck et al May 10, 1932 1,920,307 Hechenbleikner et al. Aug. 1, 1933 

1. IN THE METHOD OF RECOVERING NITRIC AND HYDROFLUORIC ACID VALUES FROM A WASTE PICKLE LIQUOR SOLUTION CONTAINING NITRATES AND FLUORIDES IN SOLUTION, THE STEPS OF: DISTILLING THE WASTE PICKLE LIQUOR SOLUTION AT ABOUT 150*F. AND THREE INCHES OF MERCURY TO DRIVE OFF A MAXIMUM OF WATER WITHOUT SUBSTANTIAL LOSS OF ACID RADICAL, ADDING SULFURIC ACID TO THE CONCENTRATED LIQUOR FROM SAID DISTILLATION STEP, FURTHER DISTILLING THE MIXED PRODUCTY AT 150*F. AND ONE INCH OF MERCURY TO DRIVE OFF NITRIC AND HYDROFLUORIC ACID WHILE FORMING A 25% SOLIDS SLURRY OF SULFATE SALTS IN SULFURIC ACID, SEPARATING THE SULFATE SALTS FROM THE SULFURIC ACID SLURRY, SEPARATING THE SULFATE SALTS FROM THE SULTHE SEPERATION STEP THROUGH SAID LAST TYPE MENTIONED DISTILLATION PROCESS, AND CONDENSING THE VAPORS DERIVED FROM THE FIRST AND SECOND DISTILLATION STEPS. 